The present invention relates to a ceramic capacitor and method for manufacturing same; and, more particularly, to a ceramic capacitor having prolonged lifetime by using a precise control of a dielectric layer composition and a manufacturing method therefor.
Generally, a ceramic capacitor includes a chip-shaped sintered body and a pair of electrodes formed at two opposite sides thereof. In case of a multi-layer ceramic capacitor, the sintered body is generally made of alternately laminated dielectric layers and internal electrodes. Every two neighboring internal electrodes face each other through a dielectric layer disposed therebetween, and are electrically coupled to different external electrodes, respectively.
The dielectric layer is formed of a reduction-resistant dielectric ceramic, which includes ceramic grain primarily composed of BaTiO3, and an additive having a glass component serving to combine the ceramic grains. The internal electrodes are made of sintered conductive paste primarily composed of, e.g., Ni metal powder. Sintering as defined herein represents a process in which individual particles are densified through modification and bonding below melting point thereof to have a poly-crystalline structure in a shape of mass.
The sintered body is made by performing removal of binder from alternately laminated ceramic green sheets and internal electrode patterns, sintering in a non-oxidizing atmosphere at a high temperature of about 1200xcx9c1300xc2x0 C., and thereafter re-oxidizing under a mild oxidation condition.
If a ratio of Ba/Ti(A/B) of BaTiO3 contained in the dielectric layers is equal to or less than 1.000 and sintering is performed in a reducing atmosphere, the sintered product does not function as a capacitor, since the constituents of the dielectric ceramic become semi-conductive during sintering and thus insulating properties thereof is deteriorated. To improve reduction-resistant properties of the dielectric ceramic, a ratio A/B of BaTiO3 is required to be greater than 1.000. For making A/B greater than 1.000, it has been proposed to put an A-site component such as barium, strontium, and calcium greater than a stoichiometric ratio.
However, when sintering the dielectric ceramic having thus enhanced reduction-resistant characteristics, the A-site component of the perovskite crystal structure diffuses to grain boundaries so that the ratio A/B of ceramic grains becomes lowered. Therefore, reduction-resistance of the dielectric ceramic is deteriorated and oxygen deficiencies increase, resulting in a lifetime, i.e., a reliability, of a ceramic capacitor to be degraded.
It is, therefore, an object of the present invention to provide a ceramic capacitor and a method for the manufacture thereof, which has a prolonged lifetime and high reliability, wherein an A-site component of the perovskite crystal structure is prevented from diffusing into grain boundaries and thus the reduction of the ratio A/B in ceramic grains is effectively suppressed and the reduction resistance thereof is guaranteed.
In accordance with one aspect of the present invention, there is provided a ceramic capacitor having at least one dielectric layer and at least two electrodes having the dielectric layer therebetween, wherein the dielectric layer is formed of a dielectric ceramic having a sintered ceramic grain of a perovskite crystal structure in a form of ABO3, a ratio A/B of an outer portion of the ceramic grain is greater than that of an inner portion of the ceramic grain.
In such structure, the A-site component of the perovskite crystal structure will not be diffused to grain boundaries and reduction-resistance of the outer portion of a ceramic grain can be improved. Accordingly, the ceramic capacitor in accordance with the present invention has an improved reduction resistant dielectric layer, which gives rise to prolonged lifetime, and improved electric characteristics such as insulating resistance.
Herein, a dielectric ceramic of the present invention is preferably BaTiO3 or SrTiO3 based ceramic. However, other alternative dielectric ceramic may also be used if it is composed of sintered ceramic grains having the perovskite crystal structure.
The drawing of the invention shows ceramic grains, outer portions thereof, inner portions thereof, and grain boundaries of a sintered body. The outer portion of a ceramic grain indicates a portion of the ceramic grain from the outer surface toward the center thereof up to about 10 nm in depth and the inner portion of a ceramic grain represents a portion thereof inside the outer portion. The outer portion of a ceramic grain does not refer to a part of the grain boundary but a portion inside of the ceramic grain. A ratio of A/B, e.g., of BaxTiyO3, denotes molar ratio x/y of Ba and Ti.
In the perovskite crystal structure of the present invention, a ratio A/B of the outer portions of ceramic grains composing a sintered ceramic body is greater than that of the center portions thereof. In such a structure, an A-site component of the perovskite structure would not diffuse to grain boundaries. The ratio A/B of the outer portions of the ceramic grains is preferably to be within a range of about 1.000 less than A/Bxe2x89xa61.015. If the ratio A/B is equal to or lower than about 1.000, reduction-resistance is reduced and required IR(insulation resistance) lifetime is not achieved, thereby deteriorating the reliability. On the other hand, if the ratio A/B is greater than about 1.015, required sintered features and electrical characteristics or required growth of grain and electrical characteristics cannot be achieved. However, within such range of 1.000 less than A/Bxe2x89xa61.015, required electrical properties can be achieved.
It is also preferable that an amount of an A-site component ranging from about 0.05 to 0.1 mole per 100 moles of a primary component forming the ceramic grain is included in an additive containing a glass component to be used in combining ceramic grains. If the A-site component is included less than about 0.05 mole, the A-site component diffuses from the outer portions of the ceramic grains into the grain boundaries and the ratio A/B at the outer portions is lowered, which reduces reduction-resistance, thereby deteriorating the reliability. If the A-site component is included more than about 0.1 mole, the A-site component becomes a surplus and as a result the ratio A/B of the ceramic grains in the outer portions exceeds 1.015, thereby making it impossible to get the required growth of grain and electrical properties. On the other hand, if the A-site component is included within a range from about 0.05 to 0.1 mole, the diffusion of the A-site component from the ceramic grains into the grain boundaries is suppressed and the ratio A/B of ceramic grains is not allowed to be lowered, which yields the ratio A/B of the outer portions to be greater than that of the inner portion and also the ratio A/B of the outer portions to be within the range of about 1.000 less than A/Bxe2x89xa61.015, thereby enabling the required electrical properties to be obtained.
In accordance with another aspect of the invention, there is provided a method for manufacturing a ceramic capacitor including the steps of making unsintered ceramic powder, forming ceramic green sheets by mixing the uncalcined ceramic powder and an organic binder, printing internal electrodes on the ceramic green sheets to provide electrode printed green sheets, laminating the electrode printed green sheets, cutting the laminated ceramic green sheets according to the printed internal electrodes pattern to provide chip-shaped laminated bodies, and sintering the chip-shaped laminated bodies, wherein the unsintered ceramic powder includes a primary component of a perovskite crystal structure in a form of ABO3 and an additive containing an A-site component of the perovskite crystal structure.
The unsintered ceramic powder is e.g., BaTiO3 and SrTiO3 family, but other alternative ceramic powder that can form a sintered ceramic body having perovskite crystal structure may be used.
Further, as an additive having, e.g., SiO2, Li2O, B2O3 or a combination thereof as a main component can be used, but other alternative may be included in the additive.
It is preferable for an amount of the additive to be ranged from about 0.1 to 1.0 part by weight with respect to 100 moles of a primary component forming the ceramic grains. If the amount of the additive is less than about 0.1 part by weight, a required growth of grain and electrical properties cannot be obtained, whereas if the amount of the additive is greater than 1.0 part by weight, a growth of grain is hard to control for obtaining the required electrical properties or excessive growth of grains may occur, resulting in degraded reliability. However, the amount of additive ranging from about 0.1 to 1.0 part by weight makes it possible to obtain the required electrical properties.
Further, one or more components selected from the group consisting of barium, calcium, and strontium may be used as the A-site component included in the additive, but other alternative material may also be used.
It is also preferable that an amount of an A-site component ranging from about 0.05 to 0.1 mole per 100 moles of a primary component forming the ceramic grain is included in an additive containing a glass component to be used in combining the ceramic grains. If the A-site component is included less than about 0.05 mole, the A-site component diffuses from the outer portions of the ceramic grains into the grain boundaries and the ratio A/B at the outer portions is lowered, which reduces reduction-resistance, thereby deteriorating the reliability. If the A-site component is included more than about 0.1 mole, the A-site component becomes a surplus and as a result the ratio A/B of the ceramic grains in the outer portions exceeds 1.015, thereby making it impossible to get the required growth of grain and electrical properties. On the other hand, if the A-site component is included within a range from about 0.05 to 0.1 mole, the diffusion of the A-site component from the ceramic grains into the grain boundaries is suppressed and the ratio A/B of ceramic grains is not allowed to be lowered, which yields the ratio A/B of the outer portions to be greater than that of the inner portion and also the ratio A/B of the outer portions to be within the range of about 1.000 less than A/B less than 1.015, thereby enabling the required electrical properties to be obtained.